The present invention relates to a method for classifying objects detected by a vehicle mounted radar located in front of said vehicle for transmitting a radar microwave signal and for receiving reflexes thereof from objects positioned in front of said vehicle. The invention also relates to a corresponding device for performing said method.
Radar systems mounted on vehicles such as oars, buses, lorries and the like are today being developed. Such systems aim to support the driver of a vehicle by providing features such as cruise control, collision warning and collision avoidance based upon the detection of vehicles, obstacles and other objects in front of the vehicle.
A problem in such a vehicle mounted radar is to distinguish xe2x80x9charmlessxe2x80x9d overhead objects in front of the vehicle, such as overhead road signs and overhead bridges crossing the travel path of the vehicle, from ground level objects, such as cars, sideway barriers and other obstacles in front of the vehicle which the vehicle must stay clear of in order to avoid collision. As is understood, the terms ground level and overhead are used herein to define the level at which the vehicle is traveling and a level lying sufficiently above said ground level, as compared to the height of a vehicle, to avoid collision, respectively.
In horizontally scanning vehicle radars according to prior art, which does not provide elevation resolution, the approach to solve this problem has been to use a vertically very narrow radar beam. The vertical beam width is then simply chosen small enough to make overhead objects fall outside the beam envelope and thus not giving rise to any reflexions. However, the use of a very narrow beam has the disadvantage that the beam may be point in a to high or to low direction when the road is undulated or when the vehicle is heavily loaded.
An object of the invention is to provide a reliable, cost effeicient and uncomplicated way of distinguishing overhead objects from ground level objects.
According to the present invention, said object is achieved by a method of the kind mentioned in the introduction, comprising the steps of: tracking a radar reflex from an object located in front of said vehicle as the calculated distance from said vehicle to said object, as derived based upon the time of flight of said radar reflex, decreases; monitoring changes in the amplitude of said radar reflex from said object as said calculated distance from said vehicle to said object decreases; and classifying said object as an overhead object or a ground level object based upon the said changes in the radar reflex amplitude.
The invention is based upon the inside that, as the vehicle approaches an object in front, the amplitude of the radar reflex or echo from said object will show different characteristics based upon whether or not the object is a ground level object or an overhead object. Detection of these different behaviors may thus be used to correspondingly classifying the object sa a hazard or not.
According to an embodiment of the invention, the classification of an object is based upon whether or not a decrease in the monitored amplitude, occuring as the distance between said vehicle and said object decreases, is larger than a predefined amplitude drop. This embodiment is thus based upon the insight that, as the vehicle approaches an object in front, an overhead object will gradually move away from the center of the radar beam, thus gradually reflecting lesser and lesser of the radar beam back to the vehicle, resulting in a decreasing amplitude, whereas a ground level object in front of the vehicle will essentially remain more or less close to the center of the beam and hence show a non-decreasing reflex.
Preferably, the currently received reflex amplitude is compared to an average amplitude value, as averaged during a decrease in the distance between said vehicle and said object. The classification of the object as an overhead object is than made if the difference between the current value and the average value is larger than a predefined amplitude difference.
According to a second embodiment of the invention, the classification of an object is based upon the tracking of a multipath effect associated with said object reflex. As a radar signal is transmitted towards and reflected from an object, the signal may use two different travel paths. One path is the straight line between the vehicle radar and the object. The other path uses the road as a reflecting surface, whereby the signal travels downwards in a first direction and is reflected against the surface of the road to then travel upwards in a second direction to reach the target. Depending on the difference in travel distance between the two paths, reflexes associated with the different paths will interfere with each other in a destructive or non destructive manner.
As the vehicle approaches an object in front of the vehicle, the multipath effect will provide an alternatively destructive and non destructive interference, thus giving rise to a corresponding alternately decreasing and increasing amplitude of said reflex.
As will be discussed more fully below the reference to the accompanying drawings, the period (i.e. period as calculated in the distance between the vehicle and the object) of said alternately incresing and decreasing amplitude will depend upon the objects altitude above the read surface. At any given range in the distance between the vehicle and an object, a reflex from a ground level object will show a greater distance between amplitude dips, i.e. will fluctuate more slowly, whereas a reflex from an overhead object will show a shorter distance between the amplitude dips, i.e. will fluctuate more rapidly. According to the second embodiment, the object is classified as an overhead object or a ground level object based upon the detected period or space frequency of the periodic variation in the amplitude.
Preferably, in this second embodiment, said object is classified as an overhead object based upon whether or not the variance of the received reflex amplitude, preferably normalized using the mean value of said reflex amplitude, is larger than a predefined value.
Alternatively, the space frequency content of a periodically varying reflex amplitude is analyzed and said object classified as an overhead object if the content of space frequencies higher than a predefined space frequency is larger than a predefined value.
In this content, it is understood that the terms period, frequency, and so on, refer to the variations in said amplitude as expressed in spatial terms (i.e. with respect to the distance between the vehicle and the object).
In another embodiment, the different embodiments and alternatives described above is combined into one single process, wherein the different embodiments cooperate with each other based upon which embodiments that provide the most reliable result at different distance intervals.
As is understood, the method described above is preferably realized using state of the art electronic circuits and/or programmable processors, such devises incorporating the method of course residing within the scope of the invention. A great advantage of the invention is that the provision of a software solution will significangly reduce costs as compared to hardware solutions of solving the above mentioned problem.
Also, it is understood that the fact that an object is classified as an overhead object or a ground level object does not automatically mean that the radar, vehicle or driver immediately is supposed to act upon such a classification. A vehicle radar system using the invention will generally include decision rules stating for example within which distance intervals that a decision shall or may be made, said distance intervals generally being related to the speed of the vehicle and the relative speed of the object, which degree of certainty that is needed for a classification to be regarded as confirmed, and so on.